Triangulation cameras are adapted for illuminating an object and sensing reflection light reflected from the objects surface. The triangulation camera includes an imaging optic for imaging illumination light onto the object and for imaging reflection light reflected from the object on a detector camera. Triangulation cameras are simply used for obtaining an enlarged image of the objects surface or, with a more complex structure, for investigating (measuring) the shape of the objects surface with a pattern projection method (see textbook “Bildverarbeitung and optische Messtechnik in der industriellen Praxis” pp. 124, Bernd Breuckmann, 1993 Franzis-Verlag GmbH, Muenchen, Germany).
Most conventional triangulation cameras for small objects have a telecentric optical configuration providing a magnification being independent of an object distance of the camera. With small triangulation angles and low numerical apertures such systems allow single shot measurements of rather steep surfaces as e.g., a tooth which is prepared for insertion of an inlay. The telecentric optical configuration requires a complex imaging optic, which is constructed with multiple imaging lenses. The measurement field of such a telecentric triangulation measurement system is always lower than the size of the lens, which limits the application of telecentric triangulation systems to small objects.
The imaging optic can be simplified if a non-telecentric optical configuration is provided (see US 2004/0156626 A1). However, this results in a reduced imaging quality which may be unacceptable for precision applications of the triangulation camera. If the shape of the object's surface is to be reconstructed, e.g., for obtaining input data for prototyping a tooth filling, imaging precision down to a range of 5 μm to 10 μm is required.
For the simple monitoring purpose, the illumination light and the reflection light may travel with opposite directions along one common beam path. For the pattern projection method, the direction of illumination light must deviate from the direction of collecting the reflection light. In this case, additional requirements result for the optical set-up and in particular for the imaging optic as the illumination light and the reflection light do not travel along one beam path through the imaging optic. In particular, a telecentric optical configuration yielding oblique-angled beam paths is necessary, wherein optical axes of telecentric apertures are with displaced relative to the optical axis of the imaging optic.
The telecentric optical configuration, which is described with further details below, has an essential limitation in terms of the large light intensity loss at telecentric apertures. Therefore, imaging with conventional triangulation cameras can be deteriorated by scattering light, in particular by surface reflections at lens surfaces of the imaging optic.
Conventional approaches for suppressing surface reflections in standard cameras are based on anti-reflection coatings on lens surfaces or scatter light shielding apertures. It has been found in practice, that these techniques are not sufficient for triangulation cameras. Another conventional approach for suppressing surface reflections in special cameras or illumination devices is based on tilting lenses relative to the optical axis of the imaging optic. As an example, US 2006/0227,435 (or corresponding DE 103 164 16 A1) discloses an ophthalmologic camera for retina imaging having a main optic with pairwise tilted lenses. Further examples of using tilted lenses are described in U.S. Pat. No. 4,415,239 (or corresponding DE 31 43 137 C2) or U.S. Pat. No. 4,730,910 A. All the conventional techniques using tilted lenses for scatter reduction do not use telecentric optical configurations. Therefore, they cannot be used for triangulation measurement of steep surfaces as in dental cameras.